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Is this a tachometer?

Great! And you have a servo amp which accepts the tachometer wires.

Note that the easiest thing to get wrong is to wire the tach (or the motor) wires backwards. If that is done, it will shoot to full speed on one direction or the other no matter what the command voltage is.

Just test this with no mechanical connection to the machine to make sure it is right before you let the amp and motor run the machine.

If you have a lab type power supply which can put out any voltage between 0V and 10V, you can run at any speed within the motor's range. If the amp is not the one which came with the motor and machine, you may need to adjust the feedback gain so your 10V is truly the maximum RPM you want. The motor's label should tell you both the maximum speed *it* can take happily, and the volts/1000 RPM from the tach, so you can use a DC meter on the tach leads at the amp to see what speed you are really getting.

Anyway -- the lab supply will give you all possible speeds in

*one* direction, and you will have to swap ground and + to the servo amp inputs to get it to run in the other direction.

Or -- you could set up a pot between +10V and -10V with either a pair of resistors defining a 0 voltage reference and use the pot to get all speeds within the range of the motors.

Of course, again while playing with this, you want the toothed belts off the motors so you don't drive anything hard against a stop.

Set up the travel limit switches to shut down the amp if it hits one of them, and make sure that always works before you let the motors actually move the machine. (This applies, of course, whether you have tach feedback or not. :-)

I would suggest using relays on the limit switches, so each one will stop the servo motor moving in that direction, but allow moving in the other direction, but other contacts will also hit the E-stop on the controller as well, since if you hit a limit switch you want to stop all other motion as well. For convenience, given how buried some of the belts are, you might want to provide a pair of low voltages (perhaps +/-

0.5V or so) to feed to the servo amps to back off a limit switch while the computer is shut down. The Anilam conversion which I used at work had handwheels, but your (and my) Bridgeports do not. If you can find them, set up military style pull-to-unlock toggle switches (the kind which arm weapons on aircraft) for connecting the slow motion voltages into the servo amps. This way, someone has to *think* before they interrupt the control signals to the servo amps.

Enjoy, DoN.

Reply to
DoN. Nichols
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Did you resolve it?

Wes

-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller

Reply to
Wes

brushes. That is

Wes, yes, I am 100% certain that it is a tachometer. 9.5 volts per

1000 RPM.
Reply to
Ignoramus10537

brushes. That is

Interesting. The 4 brushes seem like overkill for a tach. I wonder why a two brush design wasn't good enough. EE's, got an explanation?

Wes

Reply to
Wes

Yup, the big problem with DC tachs is keeping the brushes in good contact with the commutator. My guess is they use an odd number of slots/windings/commutator segments. With only two brushes, you'd still have one brush jumping across commutator segments every small amount of rotation. With 4 brushes, paired with their opposite brush as you do with a 4-pole DC motor, then there is always one of the pair making contact with the commutator. So, this should greatly reduce brush noise and produce a much cleaner tach signal. Some small tachs use a silver-plated commutator and sliver-wire brushes. These have limited life, however, like the Maxon motor/tachs.

Jon

Reply to
Jon Elson

brushes. That is

Lower contact noise from the commutator. This allows for a higher degree of accuracy in the control loop.

Reply to
Michael A. Terrell

brushes. That is

My guess is a slight overlap of switching time between two sets of brushes so one switches while the other makes full contact to minimize glitches in the feedback signal -- which could cause glitches in the speed of the motor, and mar the finish of the workpiece.

Enjoy, DoN.

Reply to
DoN. Nichols

brushes. That is

Some years back a mate of mine mentioned he was involved in an instrumentation project on a diesel engine in a bus or truck and they used a synchronous motor, as found in mains timers etc, for the tachometer. IIRC the AC generated was rectified but I can't recall what they did with it after that, maybe a frequency to voltage converter. Do you have any ideas as to the benefits or disadvantages of this sort of tachometer. I can see that there would be no contacts so possibly reduced noise but not sure what effect methods of converting the AC to a DC voltage proportional to RPM would have in the system. I have put a synchronous motor onto a scope and spun it to generate a AC waveform but not tried it beyond that.

Reply to
David Billington

brushes. That is

You could use either a frequency to voltage converter, or a simple counter circuit. The F/V converter is a simple, single chip design, and cheap. The counter gives the option for accurate speed warnings or overspeed shutdown. Cost is probably five times the F/V converter and a meter movement.

Reply to
Michael A. Terrell

That makes a lot of sense to me. Thank you for the explanation.

Wes

Reply to
Wes

[ ... ]

The DC tach provides pretty much a constant voltage for a given speed -- even at a near standstill. I would expect the frequency of the generated AC to be so low that even rectified and filtered, you would get variation in the output voltage for a full rotation with a constant speed.

And with rectified and filtered AC output, you would have much slower response to variations in actual motor speed, so the ability to control the speed would suffer.

I believe that the AC servo motors have a pair of Hall effect sensors (at 90 degree spacing) and a rotating permanent magnet, and analyze the two signals to determine the actual physical position, and from rate of change in that determine the speed. A lot more electronics to do the same thing -- but the advantage is that there are no brushes to wear out.

Enjoy, DoN.

Reply to
DoN. Nichols

typically not an AC generator but a selsyn for this application (did I spell that right? special variant of a synchro resolver)

Reply to
Bill Noble

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